The present invention relates to the field of power supplies, and in particular, to a method and apparatus directed to a multiple output power converter with higher precision on matching output voltages.
Power converters are employed in a wide variety of electronic systems, including personal computers, cable modems, disk drives, calculators, televisions, test-equipment, and hi-fi equipment. A power converter, also called a power supply, is a device for the conversion of available power with one set of characteristics to another set of characteristics. A power converter may be used to produce a regulated (or controlled) output voltage from an unregulated input voltage. The regulated output voltage may have a magnitude and possibly a polarity that differs from the input voltage. For example, a 120 V ac utility voltage may be rectified or converted to produce a dc voltage of about 170 V. A dcxe2x80x94dc converter may then be employed to reduce the voltage to a regulated 5 V.
Because of its circuit simplicity and relative low cost, a flyback type of converter has become a favored topology for high-voltage power supplies such as televisions and computer monitors. The flyback converter topology also finds wide appeal in switching power supplies in the 50-100 watt power range.
An example of a conventional two-output flyback converter (100) is shown in FIG. 1. As shown in the figure, the conventional two-output flyback converter (100) includes a primary transformer winding (L11), a switch device (SW11), a secondary transformer winding (L12), a tertiary transformer winding (L13), a primary output diode (D11), a secondary output diode (D12), a primary output capacitor (C11), and a secondary output capacitor (C12).
In FIG. 1, the primary transformer winding (L11) is connected between nodes N101 and N104. The switching device (SW11) is connected between node N104 and a circuit ground potential (GND). The secondary transformer winding (L12) is connected between node N102 and a circuit ground potential (GND). The primary output diode (D11) is connected between node N102 and node N103. The primary output capacitor (C11) is connected between node N103 and a circuit ground potential (GND). The tertiary transformer winding (L13) is connected between node N105 and node N107. The secondary output diode (D12) is connected between node N105 and node N106. Node N106 is connected to a circuit ground potential (GND). The secondary output capacitor (C12) is connected between node N107 and node N106. A core (F11), such as a ferrite core, is located between the primary transformer winding (L11), the secondary transformer winding (L12), and the tertiary transformer winding (L13).
Although the transformer windings (L11-L13) and the core (F11) appear similar to a transformer, it is more descriptively referred to as a xe2x80x9cthree winding inductor.xe2x80x9d Unlike an ideal transformer, the current does not flow simultaneously in the first and second (or third) windings of the conventional two-output flyback converter (100). Instead, the flyback converter""s magnetizing inductance assumes the role of an inductor and a magnetizing current is switched between the primary transformer winding (L11), the secondary transformer winding (L12) and the tertiary transformer winding (L13), during the flyback""s operation.
In operation, an input voltage (Vin) is coupled to node N101, and the flyback converter (100) provides a primary output voltage (Vo11) and a secondary output voltage (Vo12) at nodes N103 and N107 respectively. The primary (Vo11) and secondary (Vo12) output voltages are coupled to a primary and secondary load (not shown). The flyback converter has two operating modes corresponding to the operation of the switching device (SW11).
During the first mode of operation of the flyback converter (100), the switching device (SW11) is closed (the xe2x80x9conxe2x80x9d period). The winding polarity of the transformer ensures that the output diodes (D11) and (D12) are reverse-biased so that no transformer secondary current flows through the secondary transformer winding (L12). The primary transformer winding (L11) functions as an inductor, connected through node N101 to the input voltage (Vin) and producing a primary current. The primary current rises linearly in the primary transformer winding (L11) during this period. The transformer is designed to have a high inductance so that energy is stored in the magnetic field. The output capacitors (C11) and (C12) act as reservoirs (having been charged during the xe2x80x9coffxe2x80x9d periods) maintaining the voltages across the loads (not shown).
In the second mode of operation (the xe2x80x9coffxe2x80x9d period), the switch circuit (SW11) is opened and the primary current ceases to flow in the primary transformer winding (L11). The magnetizing current is then referred to the secondary transformer winding (L12) and the output diodes (D11) and (D12) now become forward biased. Thus, energy stored in the magnetic field of the converter during the xe2x80x9conxe2x80x9d period of the switching device (SW11) is transferred to the output loads (not shown) creating the first and secondary output voltages (Vo11 and Vo12).
The present invention is directed to provide a method and apparatus that produces high precision output voltage matching in a multiple output power converter. In a conventional multiple output power converter, such as a flyback converter, poor output voltage precision often is a result of the non-ideal characteristics of the components used in the circuit. The present invention minimizes the effects of the non-ideal characteristics of the components by the incorporation of transfer capacitance circuits. As a result, the output voltages are closer together in their magnitudes, resulting in higher precision in matching output voltage.
In accordance with one embodiment of the present invention, an apparatus is directed to producing multiple output signals from an input signal. The apparatus includes inductive windings, transfer capacitance circuits, rectifier capacitance circuits, output capacitance circuit, and a switching circuit. In the apparatus, a first inductive winding is magnetically coupled to a second inductive windings. A first transfer capacitance circuit is coupled to the first and second inductive winding. A first rectifier circuit is coupled to the second inductive winding and a first output terminal. A first output capacitance circuit is coupled to the first output terminal and a circuit ground potential. A second transfer capacitance circuit is coupled to the first inductive winding and a third inductive winding. A second rectifier circuit is coupled to the third inductive winding and the circuit ground potential. Additionally, a second output capacitance circuit is coupled to the second output terminal and the circuit ground potential. In the apparatus, the first and second transfer capacitance circuits store energy in response to the input signal when the switching circuit is in a closed position. The first and second transfer capacitance circuits transfer energy through the first and second output terminals respectively when the switching circuit is in an open position. One of the multiple output signals is associated with the first output terminal and a second of the multiple output signals is associated with the second output terminal. The output signals associated with the first and second output terminals have substantially the same magnitude. Moreover, the first, second, and third inductive windings may be wound on a common core.
The apparatus above can be extended by further including at least one additional circuit that is arranged to provide an additional one of the multiple output signals. Each of the additional circuits includes an additional inductive winding, an additional transfer capacitance circuit that is coupled to the first inductive winding and the additional inductive winding, and an additional rectifier circuit that is coupled to the additional inductive winding and an additional output terminal. Further, each additional circuit includes an additional output capacitance circuit that is coupled to the additional output terminal and the circuit ground potential. The additional transfer capacitance circuit is arranged to store energy in response to the input signal when the switching circuit is in the closed position. The additional transfer capacitance circuit is arranged to transfer energy through the additional output terminal when the switching circuit is in the open position such that a third one of the multiple output signals is associated with the additional output terminal.
In accordance with yet another embodiment of the present invention, a method is directed to provide output signals to a first and second output load circuit in response to an input signal. The method includes closing a switching circuit during a first operating mode and opening the switching circuit during a second operating mode. The first operating mode includes charging a first inductive winding in response to the input signal. The first operating mode further includes charging the second inductive winding through a first transfer capacitance circuit. Included in the method is coupling of magnetic energy from the first inductive winding to a second inductive winding. Further included in the first operating mode is the charging of the third inductive winding through a second transfer capacitance circuit. Moreover, the first operating mode includes providing an output signal to the first output load from a first output capacitor, and providing a second output signal to the second output load from a second output capacitor and through the second inductive winding and the second transfer capacitance circuit.
The second operating mode of the method includes storing energy in the first output capacitance circuit, storing energy in the first transfer capacitance circuit, and storing energy in the second transfer capacitance circuit. The second operating mode further includes providing the first output signal to the first output load from the second inductive winding and the first transfer capacitance circuit, and providing the second output signal to the second output load from the third inductive winding and the second transfer capacitance circuit. The first one of the output signals and the second one of the output signals have substantially the same magnitudes.
In yet another embodiment of the invention, an apparatus is directed to provide a positive output signal to a first output load and a negative output signal to a second output load circuit in response to an input signal. The apparatus includes a first inductive means that stores energy in response to the input signal, a first charging means that charges the first inductive means when active, and a first capacitive means that is coupled across the first output load circuit and arranged store energy when the first charging means is inactive. The first capacitive means is directed at providing the first output signal to the first output load circuit when the first charging means is active. The apparatus further includes a second inductive means that is magnetically coupled to the first inductive means. The second inductive means of the apparatus is selectively coupled across the first output load circuit when the first charging means is inactive. Further, the apparatus includes a second charging means is charged by the first inductive means when the first charging means is inactive. The second charging means is arranged to charge the second inductive means when the first charging means is active, such that the second inductive means and the second charging means provide energy to the first output load circuit when the first charging means is inactive. Also included is a third inductive means that is selectively coupled across the second output load circuit, and a third charging means that is charged by the first inductive means when the first charging means is inactive. The third charging means of the apparatus is arranged to charge the third inductive means when the first charging means is active, such that the third inductive means provides energy to the second output load circuit and the third charging means provides energy to the second output load circuit when the first charging means is inactive. The apparatus provides energy to the first output load circuit and the energy to the second output load circuit having substantially the same magnitude.
A more complete appreciation of the present invention and its improvements can be obtained by reference to the accompanying drawings, which are briefly summarized below, to the following detail description of presently preferred embodiments of the invention, and to the appended claims.